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TOPIC: Question with a sebenza

Re: Question with a sebenza 1 year 11 months ago #4306

  • cbwx34
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BassLakeDan wrote:
I make one conclusion so far about all this: 1.) I think we might have debunked here the ‘must be operator error and you have a wire hair remaining on the blade apex’ issue.. I conclude that the thing would have never survived 75 contacts in the same area and only shown a 100 gram increase if there were serious apex issues with sharpening technique. It seems likely that things would have degraded much more rapidly and the push cut forces would have ramped up to much higher values much sooner if that was the case

Good post and testing... but I'm not sure I understand (or if you're saying...) how you came to this conclusion for Rlb's knife? I think it would also be benefical if, after the testing you just did (if you haven't done anything else to the knife), is to repeat what Rlb did... make 10 cuts thru some paper and see how the edge holds up.
Last Edit: 1 year 11 months ago by cbwx34.
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Re: Question with a sebenza 1 year 11 months ago #4309

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cbwx34 wrote:
..but I'm not sure I understand (or if you're saying...) how you came to this conclusion for Rlb's knife? I think it would also be benefical if, after the testing you just did (if you haven't done anything else to the knife), is to repeat what Rlb did... make 10 cuts thru some paper and see how the edge holds up.

The answer is because Ribs knife is a member of a set.

Well, it's the old problem: If you don't ask the right question you will get the wrong answer every time. In this case you have phrased the question (and it looks like maybe Cliff did as well..) to inquire specifically about a single member of and one occurrence in a subset of a much larger set. By phrasing any question that way it forces the answer to include the member of the subset to then become part of the answers equation. The question, properly phrased in relation to the largest set of which all are members of is: “Why are there so many reports of issues with the CRK sebenza failing to properly hold or take an edge that is equivalent to other similar knives made with the same steel?” If you ask the question like this “Why did Joe or John or Harry have trouble with the CRK sebenza failing to properly hold or take an edge that is equivalent to other similar knives made with the same steel?” then, of course Joe, John and Harry become part of the answer, when in fact they are not.

Forensic science battles with this all the time, all science does in fact. In this brief topic thread you have 3 separate reports (the Joe John and Harry’s of this case..) of trouble with the knife in question. If you look at the larger sets as seen on, for example, Blade Forums, Knife Forums, etc you will see hundreds of trouble reports that comprise a very large set group..

Anyway, I did as you requested, I cut 10 slices of copy paper with the sebbie, and then it began to fail to direct push cut at 90. I put it on the test jig and took a sampling of points across the blade and the numbers have jumped to 350-375 grams from un disturbed locations that were in the low to mid 200s. I will repeat all this on the ZT0550 as soon as I get a chance, hopefully that will be today, but only so many hours in the day for me :( .
Last Edit: 1 year 11 months ago by BassLakeDan.
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Re: Question with a sebenza 1 year 11 months ago #4310

  • CliffStamp
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BassLakeDan wrote:
The question, properly phrased in relation to ...

As an aside, that was a very nice description of the problem, very elegantly worded.

In regards to performance, one thing to keep in mind is that however Reeve is hardening the steel, S30V / S35VN etc. the difference in performance can not be that dramatic. If you look at edge retention by heavy abrasion (CATRA) or micro-loading to measure sensitivity to roll/chipping, then it takes something major to make a multiple factor.

For example Leatherman's 154CM compared to AISI420 which is a dramatic difference in both hardness (likely 5+ points) and carbide volume difference of > 15% only produces a 3X difference in edge retention and in order to even get this you have to go to extremes and wear the edge down to under 10% of initial sharpness (blunting is nonlinear so the further you go the larger the gap you will see between steels).

As strength would be linear with hardness, a 1-2 point change in hardness could produce at most < 5% of a difference in strength. Now if Reeve is leaving large amounts of retained austenite in the steel there could be a more significant effect, but again I can't see how you could even get a 100% difference due to that as you are talking about maybe a 10-15% compositional change at maximum.

One thing to be really careful about is how perceptive bias will influence people. It only takes a very small amount of people to suddenly create the idea that there is a problem, people will then mob to that like sheep and see a problem when none exists. Unless you have people actually doing blind runs, which few people do - be very very careful as Elmer Fudd notes.

Again to clarify, I am not saying I agree with Reeves decision being optimal and I have said as much on his forum - but I can't see it being that dramatic of a difference either. This is physics after all and all of these properties have bounds on them. You can't take S35VN and drop the hardness point by one and all of a sudden it is acting completely different. Note that the difference between for example Reeve and Spyderco is likely smaller than the difference in either of their knives in-batch. It is common to see HT notes claim 1-3 HRC as the goal because the batch tolerances, thermocouple tolerances, etc. . all can easily produce a difference of that size.

All of that said, I would be interested in your results. By the way it is pretty simple to add a regression line to the results of the tape and then a simple t-test to check if the force statistically changed within a significance tolerance. If you send me the XLS sheet I can add that.
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Re: Question with a sebenza 1 year 11 months ago #4311

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BassLakeDan wrote:
cbwx34 wrote:
..but I'm not sure I understand (or if you're saying...) how you came to this conclusion for Rlb's knife? I think it would also be benefical if, after the testing you just did (if you haven't done anything else to the knife), is to repeat what Rlb did... make 10 cuts thru some paper and see how the edge holds up.

The answer is because Ribs knife is a member of a set.

Well, it's the old problem: If you don't ask the right question you will get the wrong answer every time. In this case you have phrased the question (and it looks like maybe Cliff did as well..) to inquire specifically about a single member of and one occurrence in a subset of a much larger set. By phrasing any question that way it forces the answer to include the member of the subset to then become part of the answers equation. The question, properly phrased in relation to the largest set of which all are members of is: “Why are there so many reports of issues with the CRK sebenza failing to properly hold or take an edge that is equivalent to other similar knives made with the same steel?” If you ask the question like this “Why did Joe or John or Harry have trouble with the CRK sebenza failing to properly hold or take an edge that is equivalent to other similar knives made with the same steel?” then, of course Joe, John and Harry become part of the answer, when in fact they are not.

Forensic science battles with this all the time, all science does in fact. In this brief topic thread you have 3 separate reports (the Joe John and Harry’s of this case..) of trouble with the knife in question. If you look at the larger sets as seen on, for example, Blade Forums, Knife Forums, etc you will see hundreds of trouble reports that comprise a very large set group..

Anyway, I did as you requested, I cut 10 slices of copy paper with the sebbie, and then it began to fail to direct push cut at 90. I put it on the test jig and took a sampling of points across the blade and the numbers have jumped to 350-375 grams from un disturbed locations that were in the low to mid 200s. I will repeat all this on the ZT0550 as soon as I get a chance, hopefully that will be today, but only so many hours in the day for me :( .

Makes sense... although I'm not so sure that the conclusion can be so quickly made. But I see where you're coming from.

Thanks for doing the cut test... at least that relates some to Rlbs knife. This will all be even more beneficial when your tester becomes more widely used, and some of your findings can be correlated to other factors... for example right now saying a blade "failed to cut" could mean a number of things, but having an actual number would certainly narrow things down. I like where you're headed with that.
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Re: Question with a sebenza 1 year 11 months ago #4318

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CliffStamp wrote:
...
As strength would be linear with hardness, a 1-2 point change in hardness could produce at most < 5% of a difference in strength.....

Well, darn the luck I just got home very late and did not get to the ZT testing, but will for sure do it and post results tomorrow..

I know very little about steel from a material science point of view, but do I understand what you are saying here?

Is your use of the term "strength" the same term I am thinking of in this discussion here, my term is more like: "wear resistance in normal use" (wear resistance to me would include a range of factors including deformation or plasticity if the blade edge was side cut or 'torqued' against a load, abrasion resistance to hard materials, etc etc..) anyway my question is what you are saying is that wear resistance is liner related to the RC hardness scale?

I thought the effect was more logarithmic and therefore the factor becomes very significant across the range of say: 56RC to 61RC. I did not pick those numbers out of the air, as Reeve states that when they began the use of Crucible steels at their shop (around 2002 ?) they chose 56, in sharp contrast to Phil Wilson who was working with Crucible and responsible for a lot of the development and stated optimal RC to be targeted to 61. So there is a big range there. Is it just linear, is that correct? If I have a 56RC then I have to make it test RC 112 to double its wear resistance??
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Re: Question with a sebenza 1 year 11 months ago #4320

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BassLakeDan wrote:
I thought the effect was more logarithmic and therefore the factor becomes very significant across the range of say: 56RC to 61RC.

Hardness itself would have a linear change, however if the microstructure changes then the effect would not be linear with hardness. The problem is that too much is attributed to hardness when it only measures one thing which is the resistance of the steel to compression on a fairly large scale. For example 1095 and D2 at 60 HRC have the same hardness but they do not wear at all the same as the microstructure is completely different.

As manufacturers do not say exactly how they HT their steels everyone ends up guessing which is made complicated because the reported properties are from a user group which tends to be fairly biased and you can get large sheep like tendencies as there are no blind/controls being used so a lot of care has to be taken. I am not saying that the choices Reeve makes are optimal, but I would take care in heavy extrapolation from the user reports.

The question that needs to be asked is why exactly are Reeve's knives slightly softer as there are many ways that they could be that way and all will produce different micro structures. For example he could simply reduce the soak temperature, put less carbon in solution. This will end up with a perfectly fine blade but it will suffer corrosion issues because the soak was not hot enough to get the chromium in solution. In general Reeve doesn't have common reports of easy corrosion (relative) so I don't think this is the case.

He could be doing a simple air quench (not oil or positive pressure) and no below room temperature quench and if he did this then the micro-structure would be adversely effected as instead of the ideal which is martensite with less than 5% retained austenite he would have other primary structures and a much higher fraction of retained austenite and possibly even embrittlement. If he is doing this then you could see effects beyond what you would expect from the small change in hardness.

I have seen the reports where people have had blades rehardened and they report excellent results, however I am always very leery of such reports as again that could just be expectation bias. As what you are doing is fairly quantitative and not overly influenced by subjectivity I am interested in your results. I would like to see, if you have the time/inclination if you would see a different relative performance if the edge was finished with a high and low polish.

If the Sebenza has a lower relative ability to hold the low grit finish it is a fairly strong indication of a micro-structure issue as the teeth are approaching the size of the micro-structure (micron scale).
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Re: Question with a sebenza 1 year 11 months ago #4322

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CliffStamp wrote:
...

As manufacturers do not say exactly how they HT their steels everyone ends up guessing ..

The question that needs to be asked is why exactly are Reeve's knives slightly softer as there are many ways that they could be that way and all will produce different micro structures. ..
...
if you have the time/inclination if you would see a different relative performance if the edge was finished with a high and low polish. If the Sebenza has a lower relative ability to hold the low grit finish it is a fairly strong indication of a micro-structure issue as the teeth are approaching the size of the micro-structure (micron scale).

Thanks Cliff for the above explaination. I will have time today to continue my experimentationswith the ZT and may be able to re-polish the sebbie. I have equip here to take a blade only to 0.25u , and anyway I am not nearly as good at those efforts (or near as fast!) as Clay, who can polish a blade faster than you can say ready-set-go much finer than that! :ohmy:

I find it kind of interesting that you are sort saying, 'well lets be careful about putting too much on the shoulders of the physics of the steel, because we can not really account for the RC (and other differences of the material) to give us these extreme user reports. I understand your cautions here, and cbwx34 has issued cautions in the same vein, they are duly noted and I understand what you are all saying there.

:) --> However, I note that you are a 'knife steel junkie'. I see your posts about the perfomance of certain blades, I note that you recently began testing a DiamondBlade knife that uses a very sophisticated ultra high pressure forging and hardening technique to form the edge that takes the RC to +68. User reports claim 3x and 4x increase in blade life when doing field work with that knife. I pose the hypothechical: If you could have a sebbie with CBN pressure forged blade made by DiamondBlade would you not instantly grab it from my hand or would I hear you say, well "No, thanks anyway Dan keep your hybrid there, i am fully satisfied that I would only get at most a +5% increase in performance on that.." ;)

I need to clarify something about my use of the term 3x 4x. I am not using that term about the steels engineering specification: I am using it the same way DiamondBlades is using it in their advertising and product information. How we are using that term means (to use an example from DB..) "I can skin three wild boars with this blade vrs only one with Brand XYZ blade, so my DB is outperforming that blade by 3x." I am *not* saying that the engineering metals specification has gone up by 3x. For myself, I do not skin wild boars, but do carry a pocket EDC thru a fairly steady daily routine. I notice that the sebbie makes it (sometimes) thru the end of the day before I find myself looking around for a handy hone ( I tend to use a Sypderco UF beach stone that I keep in my kitchen for that...), but when I ave the ZT it might take three or four days (or sometimes more!) before I even think about dressing it. I understand that it is just all very 'seat of the pants', but oh well...
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Re: Question with a sebenza 1 year 11 months ago #4324

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BassLakeDan wrote:
If you could have a sebbie with CBN pressure forged blade made by DiamondBlade would you not instantly grab it from my hand or would I hear you say, well "No, thanks anyway Dan keep your hybrid there, i am fully satisfied that I would only get at most a +5% increase in performance on that.."

I would tend to have a high attraction to anything which is new/different as I am at heart and always will be an experimentalist. With that blade I am really curious as to how they will behave in long term wear as there is no data I can find or even extrapolate to in these cases. What we have is D2 flash-soaked to such a high temperature that massive amounts of the primary carbide have dissolved in the matrix. This allows the hardness to get up to 68 HRC and the corrosion resistance to be in the stainless class. But here is the kicker - in order to get this the carbide volume was severely reduced so where did the edge retention end up, it is harder but has less carbides.

Questions aside, I tend to prefer extreme type blades, but I know extreme type makers and those blades suit my preferences. I have for example 1095 blades at 68 HRC (small paring knife) which is just wonderful to sharpen (and of course use). It is so hard, the grain so fine, the edge is almost buttery smooth on the stone, it takes an extremely high polish without any effort and will take an edge which shaves above the skin, push cuts a tomato, etc. almost by accident. Yet, I can take the same blade and cut up sods outside with minimal damage as the hardness is right at the torsional toughness peak (pictures of all of that in the reviews).

That is an example where there is both a large change in hardness (about 10 points), a large change in microstructure (reduction in retained austenite) and very large change in torsional toughness (double if you compare the peak at about 325 F to the valley at about 500 F). If you combine all of these and the fact that the maker ground the knife accordingly to take advantage of it then you end up with a blade which can be many to one over another 1095 blade.

I notice that the sebbie makes it (sometimes) thru the end of the day before I find myself looking around for a handy hone ( I tend to use a Sypderco UF beach stone that I keep in my kitchen for that...), but when I ave the ZT it might take three or four days (or sometimes more!) before I even think about dressing it.

That is what I understood, that is really dramatic, assuming of course it is consistent on average. The first few questions that would come to mind are :

1) It is really blunting or just perceived to be - you can confirm that with the force cutting.

But there is a much harder one because again we are irrational beings :

2) Are you forcing the results because you expect them to happen? That sounds quite idiotic but it is one of the hardest problems to eliminate in research - hence double blind runs. One way to check this is to just loan the knife to a friend and ask them to return it when it needs to be sharpened and see what happens.

If it seems implied that I am using you/we in the singular sense I am not, the same thing happens to me all the time and I know it is going on, but still it isn't trivial to stop. As a recent example, I am in the middle of some edge retention work on kitchen knives. I started a group of very inexpensive blades, these are the ones marked simply "stainless", you can get them for <$10. I have another group of actual named blades like VG-10 etc. which I have called the mid-range group. The idea is to measure the edge retention of a bunch of these and then when I get another knife I can say where it is relative to the groups (I have another extreme group, the S90V class).

Now when I started the inexpensive knives I was getting a decent initial sharpness, when I started with the "better" knives my initial sharpness more than doubled. I expected this for various reasons. However after a number of posts on Spyderco's forum about placebo and bias I asked myself truly if I was not forcing the results. I was finishing on a very coarse stone (the fine side of a hardware store hone so this would be about 30 micron. If the finish is of this order should I really see any effect of aus-grain, carbide distribution, etc. . Probably not. I then did something which takes advantage of the fact that you are easily manipulate your subconscious and just kept repeating to myself how one knife in particular (one of the cheap ones) was very easy to sharpen to an extreme finish.

I then took that knife and what do you know I matched the sharpness I was getting on the more expensive knives. I then started working through all the other blades and with this bias broken all of them now fell in line and took the same quality of edge. Bias is the constant bane of any experiment and one of the things you always need to be asking yourself - is what I a seeing really due to what I think it causing it or could it be anything else. If you come up with anything you then have to find a way to make sure that is not the causal effect. At times I have always found it helpful to discuss with other people even if it is just to act as a sounding board.
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Re: Question with a sebenza 1 year 11 months ago #4329

  • BassLakeDan
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CliffStamp wrote:
.. Bias is the constant bane of any experiment ..

ain't it the truth!

Well here are some 'un-biased numbers'... I have sent this data to Cliff, who will 'crunch it' and make his own comments later.. he knows what numbers really mean, I am just a hacker, but here is my following take on the situation so far:

The ZT steels performed almost identical to the sebbie.. The basis sharpness was a little worse (higher numbers mean less sharp) than the sebbie (I think just because my 'dressing' of the blade before the test was a little hurried..) . Also per cbwx34 I re-ran the ZT through the 10 slices of copy paper. It, again, performed much like (actually exactly like!) the sebbie: i.e. after 10-15 cuts it started to fail on the 90 pushes, then I put it back on the test jig and noted the following numbers from random points along the blade: 352; 362; 344; 342; 346; 370. This is just like the sebbies numbers after the paper cutting, in other words about a 100 gram jump from baseline numbers after sharpening:

you can get the xls raw data here or just look at the following chart:



for those of you that want to see how all this stuff was collected, here is a shot of the 'test-bench'. You can build yourself your own test jig for under $50 bucks and settle all those arguments at the local beer-bar about whose blade is sharper than yours:

Last Edit: 1 year 11 months ago by BassLakeDan.
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Re: Question with a sebenza 1 year 11 months ago #4331

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So, just out of curiosity...
BassLakeDan wrote:

Keep in mind: It is important to note that push cut apex testing reveals only very limited information about real world “cutting ability”.. No matter what the results of my testing tomorrow of the ZT0550 I stand by my statement that the ZT holds up 4x better than a sebbie in the real world.

... did you change your mind? :)

I'm curious, and I know your test method is new, so you may not have this info yet, but can you give a little better idea of what a "350g test" blade can still do? You said it started to fail cutting paper... can it, for example, still shave arm hair at all, or whatever description you want to add.

Also, do you have a parts list avail. for your testing setup? I'm getting tempted...

Thanks.
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